The present invention relates in general to the field of gas metal arc welding and electrodes for gas metal arc welding. More specifically, the present invention deals with compositions of weld wires and methods of using such weld wires in the alternating current GMAW welding process, while minimizing the problems inherent in the alternating current GMAW processes.
Gas metal arc welding (GMAW) is a welding process in which an electrical arc between a filler metal and a work piece heats the filler metal and the work piece and welds them together. The filler metal in the GMAW process is usually a consumable electrode which is fed into the process as fast as it is consumed. The current passes through the electrode and the electrical arc is formed between the tip of the consumable electrode and the metal of the work piece. The GMAW welding process can be used to join two pieces of sheet metal together, as well as in many other applications. An example of a welding gun and an arrangement for GMAW is schematically shown in FIG. 1. A consumable welding electrode 14 is fed into the welding process through a welding gun 10. Electrode 14 is melted by an electrical arc 18 established between the electrode and the work piece consisting of metal sheets 11 and 13. Externally supplied gas, such as Ar, CO2 or mixtures thereof, enters the welding process through a gas nozzle 12 in welding gun 10 and shields the arc, the tip of the electrode and the pool of molten metal 15 by forming a gas shield 16. The advantages of the GMAW process are the high quality weld that can be produced faster and with very little spatter and loss of alloying elements due to the gas shield and a stable electrical arc.
The consumable electrode in FIG. 1, which is melted by the electrical arc, is transported by the arc to the work piece to serve as a filler metal. The arc produces the heat for the welding process and is maintained by the electron flow from a cathode (positive terminal) and an anode (negative terminal). In the GMAW context both the consumable electrode and the work piece can function as a cathode or an anode.
The electrical power for arc welding is obtained in two different ways. One of the ways is to generate it at the point of use, the other way is to convert it from available power from the utility line. The power conversion can involve a transformer converting a relatively high voltage from the utility line to a liner voltage for alternating current welding. Or it can involve a transformer to lower the voltage, following by a rectifier changing the alternating current to direct current for direct current welding. One of the advantages of the alternating current is cathode-related cleaning (sputtering) which removes refractory oxides from the joint surfaces, providing superior welds. In such a case, argon is the inert gas of choice for manual welding whether used with direct or alternating current.
In a straight polarity configuration the electrode is negative and the work piece is positive, which configuration is called direct current electrode negative (DCEN). In a reverse polarity configuration the electrode is positive and the work piece is negative, which configuration is called direct current electrode positive (DCEP). In a schematic illustration of a DCEP configuration in FIG. 2(a) the electron flow is directed from a negatively charged work piece to a positively charged electrode, while the flow of positively charged ionized particles of the shielding gas flows to the negatively charged work piece, bombarding it and adding to the overall heating of the work piece and causing deep penetration of the weld into the work piece. In a schematic illustration of a DCEN configuration shown in FIG. 2(b) the electron flow is directed from a negatively charged electrode to a positively charged work piece, while the flow of the ionized shielding gas flows from the work piece to the electrode. Therefore, in the DCEN configuration the heat flow is directed away from the work piece toward the electrode, resulting in a higher electrode melting rate and a lesser heating of the work piece. The GMAW process normally uses a direct current electrode positive (DCEP) configuration, which produces a stable arc and low spatter in GMAW applications the direct current electrode negative (DCEN) configuration often results in a non-stable erratic arc, sputter, produces poor quality weld, and, therefore it is rarely used.
When alternating current is used for welding, the process can be considered as a combination of DCEP and DCEN, as shown in FIG. 3(b), however, the current often can not flow smoothly through the electrode in the reverse polarity configuration due to the certain electrical characteristics of the process. The difficulty is caused by the arc being extinguished during each half-cycle as the current reduces to zero at each zero crossing point, requiring reigniting as the voltage rises again after each zero crossing. After reigniting the current increases again and undergoes the usual volts-amperes power cycle. As the current decreases again, the arc potential decreases. The greater the arc length is, the lower is the temperature of the arc gas, therefore a higher reigniting potential will be required to reignite the arc at each zero crossing. Depending upon the thermal inertia of the hot electrode terminals and plasma, it is possible for the cathode emitter to cool sufficiently approaching a zero crossing to stop the arc completely. When a welding electrode and a welding work piece have different thermal ability to emit electrons, the current will flow by different amounts during each half-cycle. In the worst case the arc may not reignite at all, if the cathode cools sufficiently and the rectification of the reverse polarity cycle causes arc to operate erratically.
The extinguishing of the arc during each half cycle and the rectification of the reverse polarity cycle have been the two main reasons weighing against commercial applicability of alternating current in GMAW welding processes, leading to such host of problems as arc rectification, arc stumbling, arc wandering and arc outages. Maintaining the arc during the zero crossing of the alternating current is quite difficult and often requires higher voltages than could be tolerated by the peripheral equipment. On the other hand, it would be desirable to use the deep penetration into the work piece occurring during the negative half cycle and cleaning occurring during the positive half cycle in an alternating current GMAW process.
Manipulating the AC waveform is one of the ways to influence the welding process and try to stabilize the arc. It would be desirable, of course, to design a waveform that increases the deposition of the metal while being adaptable to the current existing welding platforms. The conventional waves used in the SAW process, which are essentially normal sinusoidal waves, with the higher amplitude of the electron negative part of the cycle as compared to the electron positive part of the cycle. Under such operating conditions the arc usually remains erratic, lowering the deposition rate. The attempts to improve this conventional arrangement have mostly failed, because of the limitation of the wire feed speed. Also, to maintain a high deposition rate is at a lower frequency, the droplets should become larger, and, inversely, at a higher frequency AC the size of the droplets can be smaller and the transfer of the droplets will go smoother.
One of the ways to stabilize the arc in the GMAW process is to alter the composition of the wire electrode to add fluxing and alloying elements which function as arc stabilizers. Carbon steel metal cored wires for GMAW are flux-cored wires used as electrodes comprising a flux filler core encapsulated by a metal sheath. The core of the wire electrode is made of fluxing and alloying compounds, which core becomes a deposited weld material. The composition of the core determines the composition and physical characteristics of the weld metal. Generally, the compounds contained in the core are selected to function as deoxidizers, alloying elements, arc stabilizers and may provide additional shielding gas. Metal cored wires provide the ability to add various materials to the core, influencing the welding characteristics and conditions in a way that overcomes traditional known flaws inherent in the alternating current GMAW process. Therefore, it would be desirable to have an electrode wire with a core composition allowing to maintain the stability of the arc in an alternating current GMAW welding process while exhibiting the desired high deposition and fast fill characteristics.